let arr = [ 1, 3, 4, 6,7, 0, 9, -4];
  let n_evens = arr.iter().fold(0, |acc, i| acc + (i & 1 == 0) as usize);
  assert_eq!(n_evens, 4);

  fn count_if<T>(it: impl Iterator<Item=T>, pred: impl Fn(&T) -> bool) -> usize {
      it.fold(0, |acc, t| acc + pred(&t) as usize)
  }

I wrote the code that you suggested (LMK if I understood your points): https://godbolt.org/z/jW4o3cnh3

And here's the benchmark output, on my machine: https://0x0.st/8SsG.txt (v1 is std::count_if(), v2 is the optimization from my blog post, and v3 is what you suggested).

v2 is faster, but v3 is still quite fast.

So the case that you described has 2 layers. The internal std::count_if() layer, which has a 64-bit counter, and the 'return' layer of the count_even_values_v1() function, which has an 8-bit type. In this case, Clang propagates the 8-bit type from the 'return' layer all the way to the inner std::count_if() layer, which effectively means that you're requesting an 8-bit counter, and thus Clang generates the efficient vectorization.

However, say that you have the following 3 layers: (1) internal std::count_if() layer with a 64-bit counter; (2) local 8-bit variable layer, to which the std::count_if() result gets assigned; (3) 'return' layer with a 64-bit type. In this case the 64-bit type from layer 3 gets propagated to the inner std::count_if() layer, which will lead to a poor vectorization. Demo: https://godbolt.org/z/Eo13WKrK4 . So this downwards type-propagation from the outmost layer into the innermost layer doesn't guarantee optimality. In this case, the optimal propagation would've been from layer 2 down to layer 1 and up to layer 3.

Note: I'm not familiar with how the LLVM optimization pass does this exactly, so take this with a huge grain of salt. Perhaps it does indeed 'propagate' the outmost type to the innermost layer. Or perhaps the mere fact that there are more than 2 layers makes the optimization pass not happen at all. Either way, the end result is that the vectorization is poor.

> with an uint8_t local variable and size_t return value, an earlier optimisation removes the cast to uint8_t, because it only has an effect when undefined behaviour has been triggered

In this case, there is no undefined behaviour, because a narrowing cast to an unsigned type is well-defined. So, this could never have been a good explanation.